An Introduction to Environmental Chemistry

speeds up chemical reactions by supplying energy (Box 4.8). For most reactions

a 10°C rise in temperature causes at least a doubling of reaction rate. This sug-

gests that weathering in the tropics, where mean annual temperatures are around

20°C, will be about double the rate of weathering in temperate regions where

mean annual temperatures are around 12°C.

The effect of temperature is linked to the availability of water. The dry air of

hot, arid environments is an ineffective weathering agent. Vegetation and hence

soil organic matter are sparse, and this reduces the concentration of organic acids.

Moreover, close contact between rock particles and acid is prevented by the lack

of water. Short-lived rainfall events may move surface salts into the soil, but the

96 Chapter Four

Ferromagnesian

series

Olivine

(monomer silicate)

Pyroxene

(chain silicate)

Amphibole

(double chain silicate)

Biotite mica

(sheet silicate)

Felsic

series

Calcium feldspar

(framework silicate)

Muscovite mica

(sheet silicate)

Sodium feldspar

(framework silicate)

Potassium feldspar

(framework silicate)

Quartz

(framework silicate)

Increasing

temperature

of mineral

crystallization

in magma

Increasing

susceptibility

to weathering

Fig. 4.14Common silicate minerals ranked in Bowen’s reaction series. Note that minerals

formed under high temperatures with more ionic bonding are more susceptible to weathering.

Ferromagnesian—minerals which contain essential iron and magnesium. Felsic—a rock

containing feldspars and quartz.